A laser is made up of an optical gain medium and a resonant cavity, with the gain medium contained inside the resonant cavity. The resonant cavity defines characteristics of the output laser beam. External-cavity lasers typically include additional, external optical elements, defining a resonant cavity. The external cavity is typically significantly longer than the resonant cavity of the laser. The use of an external cavity increases the overall length of the laser, simultaneously reducing the divergence of the output beam and allowing better control over spatial mode and frequency selection.